The invention relates to an optical noise source comprising
narrowband light source means for generating a modulated optical signal and provided with a base input for receiving an electrical periodic signal for modulating a narrowband optical signal, and
an interference filter, based on path length difference, for receiving the modulated optical signal and for generating an optical composition signal.
Such an optical noise source is disclosed by an article "Measurement of frequency response of photoreceivers using self-homodyne method", by J. Wang, U. Kruger, B. Schwarz and K. Petermann, from "ELECTRONICS LETTERS" dated May 25, 1989, Volume 25, No. 11, pages 722, 723. This mentions narrowband light source means which comprise a laser diode (narrowband light source) for generating a narrowband optical signal. The anode (base input) of the laser diode is fed with a sinusoidal current (electrical periodic signal) with which the narrowband optical signal is modulated. Said modulated optical signal is fed to an interference filter which consists of two 3 dB couplers with, therebetween, two pieces of glass fibre having a mutual path length difference of approximately 1 km. The interference filter generates the optical composition signal which is composed of two mutually different frequency-modulated signals. As soon as a photodiode is illuminated with this optical composition signal, an electrical photodiode noise signal is produced whose (electrical) noise bandwidth is equal to the maximum instantaneous frequency difference between the two optical signals. If the spectrum of the electrical photodiode noise signal is to be as flat as possible within the chosen noise bandwidth (which is specified with the amplitude of the electrical periodic signal), the path length difference on which the interference filter is based must be considerably larger than the coherence length of the light source. Said coherence length, for the present-generation laser diodes, for example, is 10 meters and may increase to several kilometers for modern multi-section lasers.
This known noise source has the drawback that the interference filter needs to be based on a large path length difference (such as, for example, 1 kilometer).